The present invention relates to vapor compression refrigeration systems and, more specifically, to the use of passive desuperheating of the gaseous refrigerant used in the refrigeration system.
The field of vapor compression refrigeration systems includes refrigeration systems and air conditioners. Prior art closed vapor compression refrigeration systems utilize a cycle wherein the liquid refrigerant vaporizes to produce useful cooling, and where the following steps are used: (1) expansion of the liquid refrigerant (pressure reduction); (2) vaporization of the expanded liquid refrigerant in an evaporator to produce useful cooling (the refrigerant absorbs heat in this step); (3) compression of the gaseous refrigerant to form a compressed gaseous refrigerant (the refrigerant absorbs further heat in this step); and (4) condensation of the gas to condense the gaseous refrigerant back into a cool liquid refrigerant. The above steps are driven by the energy used to drive the compressor. Some prior art devices also include a reservoir, for the purpose of storing the cooled liquid refrigerant. Some prior art devices also require the use of oil to lubricate the compressor and therefore also require the use of oil separators to remove oil from the refrigerant, thus reducing contamination of the refrigeration system.
The refrigerant can absorb heat at various points in the refrigeration cycle, where the result is superheating. Superheating is the heating of vapor to a temperature much higher than the boiling point at the existing pressure. Superheating is also defined as the condition where the temperature of a vapor is greater than the saturation temperature corresponding to its pressure (Dossat, R. J. (1997) Principles of Refrigeration, 4th ed., Prentice-Hall International, Inc., p. 108, 120-123). Heat is initially absorbed in the evaporator. The vaporized refrigerant can continue to absorb heat before it reaches the compressor. Further heat is absorbed by the refrigerant during compression, such that superheated gaseous refrigerant is produced. Superheating in a refrigeration system has a number of adverse consequences, as revealed below.
Adverse Effects of Superheated Gas and Advantages of Desuperheating.
One adverse consequence of superheated gas is that contact of the superheated gas with a water-cooled condenser can result in excessive heating of the water within the condenser, with consequent undesirable deposit of scale (scaling). Water used in the condenser normally contains calcium bicarbonate, which is water-soluble. However, excess heating provokes the conversion of the calcium bicarbonate to calcium carbonate, which forms a water-insoluble deposit in the condenser (Demko et al, U.S. Pat. No. 5,509,462, issued Apr. 23, 1996; Pauling, L. (1970) General Chemistry, Dover Publications, N.Y., p. 504).
Another adverse consequence of passage of superheated gas through the refrigeration system is that it results in excessive expansion of the gaseous refrigerant. The result of this excessive expansion is that the compressor must compress a correspondingly greater volume of gas during passage of the refrigerant through the system (Dossat, R. J. (1997) Principles of Refrigeration, 4th ed., Prentice-Hall International, Inc., p. 120). A result of this need for an increased amount of compression is the utilization of extra power (Stoecker, W. F. (1998) Industrial Refrigeration Handbook, McGraw-Hill, New York, p. 72).
Oil from the compressor can mix with the refrigerant, resulting in the drawing of oil throughout the system, and in undesirable oil deposits in the condenser and evaporator. Oil in the evaporator can cause considerable loss of evaporator efficiency (Dossat, R. J. (1997) Principles of Refrigeration, 4th ed., Prentice-Hall International, Inc., p. 322-323). Another undesirable effect is loss and depletion of oil in the compressor. The undesirable effects of contaminating oil can be prevented by means of an oil separator. Oil separators are described below. Separation of the oil from the gaseous refrigerant is difficult when the mixture is relatively hot, but is easier when the mixture is relatively cool. Hence, a desupetheater can be used to cool the hot discharge gas at or prior to the position where the hot discharge gas enters the oil separator.
A further disadvantage of introducing superheated gas into the condenser is that it can result in inefficient wetting of the surface of the condenser, with consequent inefficient heat transfer (U.S. Pat. No. 1,946,328 issued to J. Neff). Hence, an advantage of desuperheating is more efficient heat transfer in the condenser.
In summary, desuperheating of hot discharge gas, prior to contact with parts of the refrigeration system downstream of the compressor might be expected to solve a number of problems, including: (1) scaling as a result of hot discharge gas in a watercooled condenser; (2) expansion of gas with the consequent need for excess power consumption due to extra work performed by the compressor, (3) utilization of heat, derived from a desuperheater, to improve the performance of oil separation devices, and (4) more efficient heat transfer in the condenser.
Desuperheaters
Some prior art desuperheaters use steam, while others describe desuperheaters that use refrigerant.
U.S. Pat. No. 4,454,720 issued to H. M. Leibowitz describes a heat pump for recovering energy from waste fluid, wherein a desuperheater sprays atomized liquid water into a flow of superheated steam.
U.S. Pat. No. 5,041,246 issued to F. E. Garrison describes a venturi for use in desuperheating in a steam generator.
U.S. Pat. No. 3,343,375 issued to L. K. Quick describes a desuperheater wherein only a portion of the superheated hot discharge gas is desuperheated. The superheated gas that leaves the compressor at output 12 and sent through line 71 is desuperheated, but the portion leaving at output 12 and sent through line 13 is not desuperheated.
U.S. Pat. No. 4,311,498 issued to D. K. Miller describes a desuperheater wherein hot discharge gas is desuperheated by coils 32 containing mechanically circulated water.
U.S. Pat. No. 5,336,451 issued to J. M. Lovick relates to the desuperheating of steam by the injection of cool water.
U.S. Pat. No. 1,946,328 issued to J. Neff describes a desuperheater for a refrigeration system wherein the desuperheater comprises a pump for actively driving cool liquid refrigerant to be used for desuperheating purposes.
U.S. Pat. No. 4,554,799 issued to F. T. Pallanch relates to the problem of compressing gaseous refrigerant, where the gas is expanded to such a great degree that multiple compressors are needed. A desuperheater is placed in the line between two successive compressors in order to improve the efficiency of the second compressor. Cool liquid refrigerant is not mixed with the superheated gas. Instead, cooling is by means of a heat exchanger. The preferred mode of delivery of the cool liquid refrigerant is a mechanical pump.
U.S. Pat. No. 4,311,498 issued to D. K. Miller describes a refrigeration system having a desuperheater positioned between the compressor and condenser, where the desuperheater is comprised of a line 34 containing circulating cold water (column 3, lines 34-36). The water appears to be circulated by means of a mechanical pump.
U.S. Pat. No. 5,150,580 issued to R. E. Hyde describes a desuperheater wherein cool liquid refrigerant leaving the condenser is actively injected into the condenser with a mechanical pump (column 6, lines 61-63). Desuperheating occurs in the condenser, and not in any portion of a line upstream to the condenser (column 6, lines 66-69).
U.S. Pat. No. 4,419,865 issued to P. G. Szymaszek describes a desuperheater wherein cool liquid refrigerant leaving the condenser is actively injected at a point in between the compressor and an oil separator using a pump 22 and motor 23.
U.S. Pat. No. 5,097,677 issued to M. T. Holtzapple describes a desuperheater where cool liquid refrigerant is introduced into the compressor, rather than at a point after the compressor. Holtzapple uses a capillary wick for introducing cool liquid refrigerant.
Other desuperheaters known in the art use booster pumps to introduce cool liquid refrigerant in order to accomplish desuperheating (Sandofsky, M. Store Equipment and Design. August 1997, p. 37). The use of a booster pump indicates that the desuperheater uses an active process for driving the desuperheater separate from the compressor used to drive the refrigeration system.
Receivers
The receiver serves as a reservoir for cool liquid refrigerant. Cool liquid refrigerant exiting from the condenser may be introduced directly into the receiver, and stored for later use in the evaporator. When the amount of gas being condensed is higher than that required by the load (the load at the evaporator), the extra condensed liquid must remain in the receiver. Conversely, if the load is increased (the load at the evaporator), more liquid than is being condensed is needed by the evaporator, and this extra liquid can be taken from that stored in the receiver. Typically, these imbalances last only a short time.
Ares, et al. described two types of prior art receivers, a flow-through receiver and a surge receiver (U.S. Pat. No. 4,621,505). In a flow-through receiver, the conduit connecting the condenser to the receiver is connected to the top of the receiver, so that all of the condensate is discharged directly into the receiver.
In a surge receiver, a pipe from the condenser passes below the receiver to the evaporator, wherein a T-joint protrudes upwards into the lower portion of the receiver, and allows the flow of liquid refrigerant into and out of the receiver (U.S. Pat. No. 4,506,523, issued to L. J. DiCarlo, et al.). The result is a stratification of liquid inside the receiver according to temperature (U.S. Pat. No. 4,621,505, column 5, lines 9-39).
A disadvantage of prior art receivers is that an overproduction of subcooled liquid can occur, with resultant xe2x80x9cloggingxe2x80x9d of the receiver. A related problem in prior art condensers is xe2x80x9cstarvingxe2x80x9d of the evaporator. xe2x80x9cLoggingxe2x80x9d means that too much liquid refrigerant is in the receiver as compared to the amount of gaseous refrigerant in the receiver. In other words, with maximal logging, the receiver contains little or no gaseous refrigerant. Logging is a problem with cool ambient temperatures (at night; in the winter). With cool ambient temperatures, the condensing pressure drops. xe2x80x9cStarvationxe2x80x9d of the evaporator means that the evaporator is not receiving liquid refrigerant in the amount needed to satisfy the load on the evaporator.
The prior art has attempted to reduce or prevent logging by flooding refrigerant into the condenser. Two different methods have been used in the prior art to prevent logging. In the first method, flooding of the condenser is accomplished by shutting down the condenser fan. Shutting down the condenser fan results in an increase in condensing temperature. Consequently, starvation of the evaporator is prevented. A problem with this method is that optimal efficiency of the refrigeration system is not obtained.
In the second method, flooding of the condenser can be accomplished by closing or shutting down a valve at the outlet of the condenser (U.S. Pat. No. 4,621,505 issued to R. A. Ares, et al.). The desired effect is flooding of the condenser, but an undesirable result of this is xe2x80x9cstarvingxe2x80x9d of the evaporator. To solve the problem of xe2x80x9cstarving,xe2x80x9d some prior art refrigeration systems restrict or close the valve at the outlet of the condenser, while at the same time introducing hot discharge gas at the top of the receiver. This hot discharge gas pressurizes the receiver, so that the expansion valve is no longer starved. When the hot discharge gas pressurizes the receiver, liquid refrigerant in the receiver is forced to exit and be transmitted to the evaporator.
A problem with the method of introducing hot discharge gas into the receiver is that optimal efficiency of the refrigeration system is not obtained. The problem is that, although the hot discharge gas pressurizes the receiver (as desired), it also has the effect of heating the liquid refrigerant in the receiver (which is not desired), and thus also has the effect of heating the liquid refrigerant that is sent to the evaporator, thereby reducing its efficiency.
Oil Separators
U.S. Pat. No. 4,506,523 issued to L. J. DeCarlo, et al. describes an oil separator that contains a centrifugal vortex. This patent points out a problem in oil separators, namely, xe2x80x9cthe cooling of separated oil below the condensing temperature of the gas refrigerant frequently produced excessive refrigerant condensation in and dilution of the oil.xe2x80x9d (column 1, lines 50-53). DeCarlo, et al. attempt to resolve this problem by use of xe2x80x9ca series of baffle or deflector plates . . . thereby assisting in . . . enhancing final separation of refrigerant vapor . . .xe2x80x9d (column 5, lines 46-54). The device in DeCarlo, et al.""s patent contains an oil reservoir, but this reservoir does not contain a heater (where the heating prevents refrigerant vapor from condensing in the oil).
U.S. Pat. No. 4,311,498 issued to D. K. Miller describes an oil separator means, wherein hot discharge gas is desuperheated by coils 32 containing mechanically circulated water, wherein oil accumulates in the lower portion of the desuperheater 30, from which it is sent to the compressor (column 4, lines 4-15). The oil separator contains no means to heat the separated oil.
Defrosters
Defrosting has been accomplished by means of: (1) electric defrosters; (2) by mixing superheated gas and desuperheated gas (from the top portion of the reservoir), and sending the mixture through the evaporator; and (3) by mixing superheated gas and cool liquid refrigerant and directing the mixture to the evaporator (U.S. Pat. No. 3,343,375 issued to L. K. Quick).
A disadvantage of using straight superheated gas for defrosting is that extreme ranges of expansion and contraction can cause breakage and leaks in the refrigeration system""s lines or tubing. A disadvantage of using a mixture of superheated gas and desuperheated gas (acquired from the upper portion of the reservoir) is that this gas in the upper portion of the receiver is in limited supply. Note that the shortage of this gas is especially severe during xe2x80x9cloggingxe2x80x9d of the receiver.
U.S. Pat. No. 5,934,297 issued to P. J. Wolff, et al. and U.S. Pat. No. 5,921,092 issued to J. A. Bahr et al describe a prior art electric defroster, where defrosting is effected by an electric heater, and not by means of hot gas.
U.S. Pat. No. 4,506,523 issued to L. J. DeCarlo, et al. describes a defroster where superheated gas is mixed with cool gas from the upper portion of the receiver.
U.S. Pat. No. 3,343,375 issued to L. K. Quick describes a defroster where a mixture of superheated gas with liquid refrigerant (from the lower portion of the receiver) is used.
U.S. Pat. No. 4,621,505 issued to R. A. Ares, et al. describes a defroster where cool gas from the upper portion of the receiver (and not mixed with hot gas) is used for defrosting (column 4, lines 2-8).
The present invention relates to the addition of desuperheating to the prior art three step refrigeration system. According to the invention the sequence becomes: (1) expansion and evaporation; (2) compression; (3) desuperheating; and (4) condensation.
The passive desuperheater of the invention comprises an inlet for superheated gas, an inlet for cool liquid refrigerant, a chamber where the superheated gas and cool liquid refrigerant contact and mix with each other, and an outlet for the desuperheated gas. In one embodiment, the superheated gas moves in an essentially straight stream, during which it is desuperheated. In another embodiment, the superheated gas moves through three nested containers, during which desuperheating occurs, removal of contaminating oil occurs, and the removed oil is heated by incoming superheated gas.
In another embodiment of the passive desuperheater, all of the superheated gas is directed into the receiver and released into the lower portion of the receiver, that is, that portion of the receiver which contains a store of cool liquid refrigerant. In this embodiment, all of the superheated gas moves through the cool liquid refrigerant within the receiver, where it is desuperheated. After rising through the cool liquid refrigerant, the gas enters the upper portion of the receiver (that portion containing only gas or vapor), and finally exits via a pipe connected to the condenser.
In a related embodiment, superheated gas is transmitted through a pipe positioned in the receiver where passage of the superheated gas through the pipe results in heat transfer through the wall of the pipe to the surrounding cool liquid refrigerant in the receiver (resulting in desuperheating), and where the desuperheated gas is released into the upper portion of the receiver at the open end of the pipe. Once released into the upper portion of the receiver, the desuperheated gas is free to exit the receiver through a line in communication with the condenser.
In another embodiment, the superheated gas travels through a closed line in the the receiver where passage of the superheated gas through the line results in heat transfer through the wall of the line to the surrounding cool liquid refrigerant in the receiver (resulting in desuperheating), and where the desuperheated gas traveling through the closed line is directed out of the receiver, and to the condenser. A separate line leads from the upper portion of the receiver to the condenser inlet.
A further embodiment of the desuperheater utilizes a shell and tube condenser. In this desuperheater, superheated gas enters the shell and tube condenser through a port located at or near the bottom of the condenser. The desuperheater comprises a narrow diameter pipe, which receives superheated gas, and a wide diameter pipe fitted over the narrow diameter pipe. The end region of the wide pipe fits loosely over the end region of the narrow pipe. The region of overlap, which involves a loose fit, allows limited entry of cool liquid refrigerant into the stream of the superheated gas. Upon limited entry of the cool liquid, and mixing with the stream of superheated gas, the superheated gas becomes desuperheated. Desuperheated gas is released at the far end of the wide pipe, where it enters the upper portion of the shell and tube condenser. All of this desuperheated gas eventually condenses in the shell and tube condenser. A line leads from the lower portion of the condenser and allows cool liquid refrigerant to flow to the evaporator.
In another alternative embodiment, the receiver may take the form of a true surge receiver. The true surge receiver comprises a receiver having a minimum subcooling valve (MSV) occurring in the line leading from the condenser outlet to the receiver. The true surge receiver further comprises a T-joint downstream of the condenser outlet and upstream of the MSV. The T-joint leads to three lines leading, respectively, to: (1) the condenser outlet; (2) the evaporator; and (3) the minimum subcooling valve (MSV). The MSV can close and block the flow of cool liquid refrigerant to the receiver, resulting in an increased flow of cool liquid refrigerant to the evaporator. The MSV can open and increase the flow of cool liquid refrigerant to the receiver. The MSV comprises a sensor which monitors the pressure and temperature of the condensed liquid in the condenser.
Broadly stated, the present invention comprises a passive desuperheater comprising a chamber having a first inlet for receiving said superheated gas, a second inlet for receiving cooler liquid refrigerant condensed by said condenser, said second inlet positioned below the outlet of said condenser to cause said liquid refrigerant to flow to said second inlet by the force of gravity, and an outlet that outputs said desuperheated gas for transmitting to said condenser, wherein said liquid refrigerant is caused to be mixed with said superheated gas in said chamber to reduce the temperature of said gas at said outlet.
It is therefore an object of the present invention is to decrease the formation of scale within the condenser.
A further object is to decrease oil contamination of the condenser, evaporator, and other parts of the vapor compression refrigeration system as a result of oil in the refrigerant. A related object of the invention is to prevent re-deposit of refrigerant in the separated oil, after initial separation of the oil and the refrigerant.
Still another object of the invention is to supply a mixture of desuperheated gas and cool gas, for use in defrosting the evaporator, while avoiding the following problems: (1) the excessive expansion and contraction of pipes that can occur with defrosters that are supplied with only superheated gas, and (2) the limited supply of cool gas which occurs in defrosters which draw their supply of cool gas from the upper portion of the receiver.
Yet another object of the invention is to improve the efficiency of refrigeration and air conditioning systems. More specifically, it is an object of this invention to drive the flow of cool liquid refrigerant into the mixing chamber of the discharge desuperheater without use of an external mechanical pump, a venturi pump, or a jet pump. Accordingly, it is an object of the invention that the force that draws cool liquid refrigerant into the mixing chamber of the discharge desuperheater results primarily from the use of gravity.
Another object of the invention is to provide simultaneous desuperheating to all of the hot discharge gas, rather than to provide desuperheating to only a portion of the hot discharge gas.
A further goal of the invention is to reduce xe2x80x9cloggingxe2x80x9d of the receiver, and to prevent xe2x80x9cstarvingxe2x80x9d of the evaporator under low condensing temperature conditions.